At level 40, you should head over to the Barbarian Agility Course. You can get there by selecting the Skills teleport, but this time choosing Barbarian Course. (Note: You must stand infront of the obstacle to attempt it)

The following map will guide you on how to complete the course:

Map of how to complete Barbarian Agility Course

Obstacle

Experience

Ropeswing

2,500

Log Balance

2,900

Obstacle Net

2,800

Balancing Ledge

2,500

Ladder

2,500

Crumbling Wall 1,2,3

7,500 (total of all 3)

Bonus Exp for completing

10,000

Total

30,700

Runs for level 99: 423

After the balance ledge every time you go down the ladder it is 2500 exp. you can keep going up and down the ladder for easy exp but without the bonus.

You can choose to keep running this until 99 aglility or you can head over to the Wilderness Aglity Course at level 60 aglility